The statement is plain wrong. Double quotation marks (or “full quotation marks”) can be used within an attribute value without problems or precautions, when normal, “curly” quotation marks are used—and the page specifically refers to them: “”. An attribute specification like content="Following foolish “SEO” instructions" would do just fine.

The author probably meant to warn against using Ascii ("straight", vertical) quotation marks. The warning is not related to meta tags in any particular way; it’s just part of general HTML syntax that the character used as attribute value delimiter cannot appear as such within the attribute value in markup.

So content="Following foolish "SEO" instructions" would result in a syntax error (detectable by a validator). But the conclusion is wrong: if you wanted to use Ascii quotation marks in an attribute, for some odd reason, you could do that e.g. writing content='Following foolish "SEO" instructions' or, alternatively, content="Following foolish &quot;SEO&quot; instructions".